linux-ext4 - Re: [RFC PATCH v3 00/18] fscrypt: key management improvements

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Message-ID: <20190220183619.GA177939@gmail.com>
Date:   Wed, 20 Feb 2019 10:36:20 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     David Howells <dhowells@...hat.com>
Cc:     linux-fscrypt@...r.kernel.org, Satya Tangirala <satyat@...gle.com>,
        linux-api@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        keyrings@...r.kernel.org, linux-mtd@...ts.infradead.org,
        linux-crypto@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, Paul Crowley <paulcrowley@...gle.com>
Subject: Re: [RFC PATCH v3 00/18] fscrypt: key management improvements

Hi David,

On Wed, Feb 20, 2019 at 06:07:22PM +0000, David Howells wrote:
> I have a couple of patches that add ACLs to keyrings, that you can find at the
> top of the branch here:
> 
> 	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=keys-acl
> 
> I have other patches that allow tags to be used as subjects in the ACL, with a
> container supplying a tag, e.g.:
> 
> 	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/commit/?h=container&id=e0fdc5613a32b9475b025f58e7a2267329b3f3a4
> 
> Might something similar be of use to you here, perhaps specifying a tag
> referring to the blockdev of interest rather than the container?
> 
> David

I don't think so.  The main point of adding keys directly to the filesystem is
that it's generally inappropriate to apply OS-level access control or
process-based visibility restrictions to fscrypt keys given that:

- The unlocked/locked status of files is already filesystem-level.
- The purpose of encryption is orthogonal to OS-level access control.

The ioctl based interface also makes it *much* easier to implement all the
semantics needed for removing fscrypt keys.

I don't see how key ACLs would be appropriate here, except that key ACLs would
allow userspace to opt-in to fixing the denial of service vulnerability where
keyctl_invalidate() only requires Search permission.  But for fscrypt that's
addressed by my proposal too, and is just one of many problems it addresses.

- Eric